In electrophotographic copying machines, copying speed has been increased year after year. In order to keep up with this technical progress, photoreceptors having a high light sensitivity and a prolonged life have been desired.
Many function-separation type electrophotographic photoreceptors comprising a plurality of members each separate functions have been proposed for improvements in electrophotographic properties such as charge retention, stability for repeated use, light response, spectral properties and mechanical strength.
These electrophotographic photoreceptors have known disadvantages in that they lack stability for repeated use or environmental stability of development contrast, they are subject to image defects such as white spot, black spot, roughness and pinhole and they exhibit so low an adhesion strength between the substrate and the light-sensitive layer that the light-sensitive layer is peeled off during use, showing insufficient durability.
In order to eliminate these disadvantages, it has been proposed to provide a resin layer as an undercoating layer between the substrate and the light-sensitive layer. As such resins there have been known polyparaxylene, casein, polyvinyl alcohol, phenol resin, polyvinyl acetal resin, melamine resin, nitrocellulose, ethylene-acrylic acid copolymer, polyamide (e.g., nylon 6, nylon 66, nylon 610, copolymer nylon, alkoxymethylated nylon), polyurethane, gelatin, polyvinyl pyrrolidone, polyvinyl pyridine, and polyvinyl methyl ether.
Further, many proposals have been made to form an undercoating layer from zirconium chelate compounds, organic zirconium compounds such as zirconium alkoxide or silane coupling agents as described in JP-A-59-223439, 61-94057, and 62-273549 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In the case where a resin layer is provided as an undercoating layer, a resin containing a relatively large amount of polar groups is mainly incorporated therein so that the volume resistivity thereof is controlled to a low level to such an extent that the electrophotographic properties are not deteriorated. However, since the volume resistivity of a resin greatly depends on the ionic conductivity and is thus extremely affected by temperature and humidity, the resin layer under low temperature and humidity or high temperature and humidity conditions exhibits a remarkably high resistivity which deteriorates the electrophotographic properties of the light-sensitive layer or a remarkably low resistivity which eliminates the desired functions of the resin layer.
Therefore, the above mentioned known resin layer can eliminate only some of the disadvantages of photoreceptors. If environmental properties are included, the effects of this approach are halved. Thus, this approach is extremely insufficient.
On the other hand, when zirconium chelate compounds, organic zirconium compounds such as zirconium alkoxide or silane coupling agents are used, the above mentioned problems can be considerably but not sufficiently eliminated. That is, these coupling agents undergo heat hardening reaction to form a film. However, if the heat hardening reaction does not sufficiently proceed, there occurs some difference in the wetting characteristics of the surface of the thin coating film which can cause a remarkable unevenness in the thickness of the thin coating film laminated with a light-sensitive layer, particularly a charge generating layer which needs to be thin, resulting in some unevenness and fog on copied images. The wetting characteristics of the surface of the coating film also change with time and thus make it very difficult to provide a charge generating layer with a uniform thickness.
As mentioned above, the conventional undercoating layer is insufficient as an undercoating layer for eliminating various disadvantages of photoreceptors. Further, the properties of the conventional electrophotographic photoreceptors are insufficient.